Operating System • Supports virtual machines o Promises each process the illusion of having whole machine to itself • Provides services: Processes o Protection o Scheduling User User o Memory management Process Process CS 217 o File systems o Synchronization o etc. OS Kernel Hardware 1 2 What is a Process? Operating System • A process is a running program with its own … • Resource allocation User User o Processor state o Sharing Process Process – EIP, EFLAGS, registers o Protection o Address space (memory) o Fairness OS Kernel – Text, bss, data, o Higher-level abstractions heap, stack User User Hardware Process Process • Common strategies o Chop up resources into small pieces and OS Kernel allocate small pieces at fine-grain level o Introduce level of indirection and provide mapping from virtual resources to physical ones Use past history to predict future behavior Hardware o 3 4 Life Cycle of a Process Context Switch • Running: instructions are being executed Process A Process B • Waiting: waiting for some event (e.g., i/o finish) Waiting Running Save context • Ready: ready to be assigned to a processor . Load context Waiting Running Create Ready Running Termination Save context . Running Load context Waiting Waiting 5 6 Overlap CPU with I/O operations Process Control Block • Schedule CPU for process B • For each process, the kernel keeps track of ... while process A is waiting for I/O o Process state (new, ready, waiting, halted) o Better utilize CPU o CPU registers (EIP, EFLAGS, EAX, EBX, …) o CPU scheduling information (priority, queues, ...) Memory management information (page tables, ...) CPU I/O CPU I/OCPU I/O o A: o Accounting information (time limits, group ID, ...) o I/O status information (open files, I/O requests, ...) B: CPU I/O CPU I/OCPU I/O 7 8 Fork Wait • Create a new process (system call) • Parent waits for a child (system call) o child process inherits state from parent process o blocks until a child terminates o parent and child have separate copies of that state o returns pid of the child process o parent and child share access to any open files o returns –1 if no children exist (already exited) o status pid = fork(); Parent #include <sys/types.h> if (pid != 0) { #include <sys/wait.h> /* in parent */ ... pid_t wait(int *status); } Child • Parent waits for a specific child to terminate /* in child */ #include <sys/types.h> ... #include <sys/wait.h> pid_t waitpid(pid_t pid, int *status, int options); 9 10 Fork Exec • Inherited: • Separate in child • Overlay current process image with a specified image file o user and group IDs o process ID o environment address space (memory) close-on-exec flag o (system call) o o file descriptors o signal handling settings affects process memory and registers supplementary group IDs o active process group ID. o o parent process ID o set-user-ID mode bit o o has no affect on file table set-group-ID mode bit o process locks, file locks, page locks, o text locks and data locks o profiling on/off/mode status o debugger tracing status o pending signals • Example: o nice value o timer signal reset times execlp(“ls”, “ls”, “-l”, NULL); o stdin o share mask o scheduler class fprintf(stderr, “exec failed\n”); o all shared memory segments o all mapped files exit(1); o file pointers o non-degrading priority o process group ID o session ID o current working directory o root directory o file mode creation mask o resource limits o controlling terminal o all machine register states o control register(s) 11 12 Exec (cont) Fork/Exec • Many variations of exec • Commonly used together by the shell int execlp(const char *file, ... parse command line ... const char *arg, ...) pid = fork() int execl(const char *path, if (pid == -1) const char *arg, ...) fprintf(stderr, “fork failed\n”); int execv(const char *path, else if (pid == 0) { char * const argv[]) /* in child */ int execle(const char *path, execvp(file, argv); const char *arg, ..., “ ” char * const envp[]) fprintf(stderr, exec failed\n ); •Also execve and execvp } else { /* in parent */ pid = wait(&status); } ... return to top of loop ... 13 14 System Summary • Convenient way to invoke fork/exec/wait • Operating systems manage resources o Forks new process o Divide up resources (e.g., quantum time slides) o Execs command o Allocate them (e.g., process scheduling) Waits until it is complete o • A processes is a running program with its own … int system(const char *cmd); o Processor state o Address space (memory) • Example: • Create and manage processes with ... o fork int main() o exec { system(“echo Hello world”); o wait } o system }Used in shell 15 16.